As is known, photographic recording materials contain so-called screening dyes or antihalation dyes to improve resolution. These dyes can be in the emulsion layer, but are preferably in light-insensitive auxiliary layers located between the support and the emulsion layer or on the back side of the support. In multilayer materials, these auxiliary layers can also be between the various emulsion layers. Without these antihalation layers, radiation reaching the photographic emulsion layer would be reflected in this layer, and image sharpness would decrease.
Because such antihalation layers impair the ultimate image by absorbing visible light, it is necessary to bleach the dyes or remove them completely after imagewise exposure. This is not a problem with aqueous-developable photographic recording materials, because antihalation dyes can be easily decolorized and/or dissolved and removed by treatment baths during or after aqueous development of the photographic recording material. However, this process cannot be used with thermally developable photographic recording materials, because the process steps with treatment solutions and the subsequently required drying should indeed be avoided in this dry development process.
Antihalation systems have been proposed in the past for thermally developable photographic recording materials that do not require added treatment solutions to bleach the antihalation dyes. Thus, U.S. Pat. No. 4,477,562 proposes fully strippable antihalation layers, which, however, can impair the storage stability of the photographic recording materials if these layers detach prematurely. The addition of various thermally active bleaching agents, such as hexaaryl bisimidazoles (U.S. Pat. Nos. 4,201,590 and 4,196,002), benzopinacols (U.S. Pat. No. 4,081,278), halogen compounds (U.S. Pat. No. 4,376,162), sydnones or iodonium salts (U.S. Pat. No. 4,581,323), and oxidizing agents (U.S. Pat. No. 4,336,323) is known. However, these can be used generally only for a limited selection of dyes or cannot achieve stable dye decolorization, so that, after a short time, background fog forms, degrading the recorded images. Special dyes that can be bleached by actinic radiation are also generally used, but these often require high temperatures or supplementary bleaching agents (U.S. Pat. Nos. 3,745,009, 4,594,312, 4,153,463, and 4,033,948).
Hence, interest continues to exist in antihalation systems for thermally developable recording materials that can be bleached without great technical expense after imagewise exposure.